TWI Industrial Member Report Summary 69/1978
By D J Abson
Recent microstructural observations strongly suggest that acicular ferrite nucleates on weld metal inclusions in steel. The consequences of these observations have been explored in some detail. From the generally observed refinement of acicular ferrite with increasing cooling rate, it is deduced that there is a range of temperatures over which the inclusions operate, i.e. some inclusions require a greater undercooling than others before they will nucleate ferrite. The effect may result from the range of inclusion sizes, although this is not necessarily so.
The model which is developed is primarily applicable to arc welds containing a substantial proportion of acicular ferrite. The ferrite laths normally grow to mutual impingement so that the lath size is determined by the number of ferrite nucleants which operate. The effect of variations in alloy content and in cooling rate on the proportions of acicular ferrite and grain boundary proeutectoid ferrite has been described. For weld metal of low oxygen content, the formation of bainite results' from the absence of efficient nucleants on which acicular ferrite laths can form or from failure of the acicular ferrite laths to grow to mutual impingement.
The formation of 'ferrite islands', ferrite side plates and grain boundary proeutectoid ferrite has also been considered.